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Abstract:

An inductive proximity switch for detecting the presence of an object in
a monitored area includes a coil (2), a pulse source (4) for supplying
the coil (2) with transmitting current pulses (S1, S2, S3) at a period
(T) larger than the duration (Ts) of the transmitting current pulses, and
a processing circuit (6) for generating an output signal (9) based on
received voltages (Ui1, Ui2, Ui3) varying in dependence of a change of
position of the object. The received voltages are induced in the coil (2)
after the duration (Ts) of a transmitting current pulse by the decaying
current which previously flows in the object due to the voltage induced
therein by the coil. The proximity switch has a suppression circuit (13,
13a, 13b, 13c) for suppressing a signal duration lower than a
predetermined perturbation time (Tc1, Tc2) in the output signal (9).

Claims:

1. An inductive proximity switch for detecting the presence of an object
in a monitored area, comprising a coil, a pulse source for supplying said
coil with transmitting current pulses at a period larger than the
duration of said transmitting current pulses, and a processing circuit
for generating an output signal based on received voltages varying in
dependence of a change of position of the object, said received voltages
being induced in said coil after the duration of a transmitting current
pulse by the decaying current which previously flows in the object due to
the voltage induced therein by said coil, wherein a suppression circuit
is provided for suppressing a signal duration lower than a predetermined
perturbation time in said output signal such that the minimum time
interval between two variations of said output signal does not go below
said perturbation time.

2. The proximity switch according to claim 1, wherein said processing
circuit is configured to generate an intermediate signal comprising two
signal states and to feed said intermediate signal to said suppression
circuit to generate said output signal.

3. The proximity switch according to claim 2, wherein at the first signal
state of said intermediate signal indicates the absence of the object
from the monitored area and the second signal state indicates the
presence of the object within the monitored area.

4. The proximity switch according to claim 2, wherein the same value of
said perturbation time is applied in said suppression circuit for said
two signal states.

5. The proximity switch according to claim 2, wherein a different value
of said perturbation time is applied in said suppression circuit for said
two signal states.

6. The proximity switch according to claim 2, wherein the two signal
states of said intermediate signal are distinguished in said suppression
circuit by means of at least two switching states of a switching circuit.

7. The proximity switch according to claim 6, wherein said switching
circuit comprises at least one multiplexer defining said switching
states.

8. The proximity switch according to claim 2, wherein said suppression
circuit comprises at least one delay circuit for delaying the
intermediate signal by the duration of said perturbation time for each of
said signal states.

9. The proximity switch according to claim 1, wherein said coil, said
pulse source, said processing circuit and said suppression circuit are
arranged in a housing being made of a metal which is not ferromagnetic
and which has a specific electric resistance higher than aluminium.

11. The proximity switch according to claim 1, wherein in a sequence of
said transmitting current pulses comprising a preceding current pulse and
a succeeding current pulse the polarity of the succeeding current pulse
is reversed with respect to the polarity of the preceding current pulse.

12. An arrangement of a proximity switch according to claim 1 and a
perturbation source, wherein the perturbation signal producible by said
perturbation source comprises a frequency of at most 50 kHz.

13. (canceled)

Description:

FIELD OF THE INVENTION

[0001] The invention relates to an inductive proximity switch for
detecting the presence of an object in a monitored area, comprising a
coil, a pulse source for supplying said coil with transmitting current
pulses at a period larger than the duration of said transmitting current
pulses, and a processing circuit for generating an output signal based on
received voltages varying in dependence of a change of position of the
object, said received voltages being induced in said coil after the
duration of a transmitting current pulse by the decaying current which
previously flows in the object due to the voltage induced therein by said
coil. The invention further relates to an arrangement and use of such a
proximity switch in conjunction with a perturbation source.

BACKGROUND OF THE INVENTION

[0002] An inductive proximity switch of that type is described in European
patent applications no. EP 0 936 739 A1 and EP 0 936 741 A1. In that
proximity switch, a coil is supplied with periodical transmitting current
pulses with a period exceeding the pulse length. By means of these
transmitting current pulses, a voltage is periodically induced in an
object to be detected. In turn, a voltage is induced in the coil
subsequently after the end of a transmitting current pulse by means of
the decaying current that flows in the object due to the voltage
previously induced therein. The useful signal is then obtained by a
suitable electronic circuit which responds to the voltage that is
received by the coil in such a manner.

[0003] A disadvantage of the proximity switch is that its magnetic circuit
is prone to couple with perturbing fields from the environment. Depending
on the frequency band, the interaction of the proximity switch with
external fields can downgrade or destruct its measuring performance. For
instance, external fields in the lower frequency range up to 500 Hz and
in the medium frequency range up to 10 kHz are often present in favored
application areas of such a proximity switch. Most critical are external
fields within the frequency band of the signal to be detected and
amplified by the electronic circuit of the proximity switch--typically in
the lower and/or medium frequency range--and an elimination of these
perturbations is essential for the basic functionality of the proximity
switch. More generally, an elimination of perturbations over the whole
frequency spectrum is highly desirable for yielding an output signal of
good quality.

[0004] The impact of external fields may be partially or fully
suppressed--primarily at the lower frequency range--by providing the
transmitting current pulses with a periodically reversed polarity, as
described in European patent application no. EP 0 936 739 A1. In this
approach, however, perturbing fields with larger amplitudes or
frequencies in the medium or higher frequency range may not be fully
eliminated by the polarity reversal.

OBJECTS AND SUMMARY OF THE INVENTION

[0005] It is therefore an object of the present invention to avoid at
least one of the above mentioned disadvantages and to propose a proximity
switch pertaining in the initially mentioned technical field, which is
adapted to generate an output signal that is less sensitive to external
perturbing fields.

[0006] Accordingly, the invention suggests that a suppression circuit is
provided for suppressing a signal duration lower than a predetermined
perturbation time in the output signal such that the minimum time
interval between two variations of the output signal does not go below
this perturbation time.

[0007] Thus, the invention proposes to adopt a perturbation time in a
suppression circuit serving as an additional fixed time parameter
supplementary to the fixed time period of the transmitting current pulses
in order to produce an advantageous output signal of the proximity
switch.

[0008] According to the invention, the combined application of these two
time parameters yields an effective setting option on the one hand for
achieving a desired insensitivity to environmental perturbing fields and
on the other hand for providing a desired degree of sensitivity for the
detection of the momentary position of the monitored object. The
invention has been carried out in particular in view of the realization
that an adjustment of the periodic time interval of the transmitting
current pulses as the only adjustable parameter may lead to an
undesirable trade-off between measurement accuracy and perturbation
insensitivity. This trade-off can be substantially reduced by the
proposed combination of at least two adjustable time parameters according
to the invention. Advantageously, the duration of the transmitting
current pulses may be additionally matched in order to optimize the
detection sensitivity and perturbation insensitivity.

[0009] Moreover, it has been surprisingly found that an advantageous
choice of a predetermined value of said perturbation time may allow an
effective elimination of external perturbation effects over a large
frequency range or substantially over the whole frequency spectrum.
Preferably, the perturbation time is fixed to be at least 0.1 ms, more
preferred at least 1 ms and most preferred at least 10 ms. In this way,
substantially the complete lower and medium frequency ranges may be
accounted for by means of said suppression circuit. It has also been
realized during the present invention that perturbations at a higher
frequency range, in particular above 100 kHz, may be intrinsically
attenuated by the electronic circuit of the proximity switch. Thus,
essentially the whole frequency spectrum may be accounted for within the
electronic circuit of the proximity switch by implementing the proposed
predetermined value of said perturbation time in the suppression circuit.
A value of at least 0.1 ms of the perturbation time may be particularly
chosen for applications in which a selected suppression of prominent
perturbations within the medium frequency regime shall be effectuated. On
the contrary, a value of at least 1 ms may be more universally applicable
and more efficient in terms of the suppression frequency range. More
specifically, a value of at least 10 ms of the perturbation time can be
especially advantageous for applications in which a plurality of
perturbations within the lower and medium frequency ranges must be
accounted for.

[0010] Preferably, the upper limit for the fixed value of the perturbation
time is chosen with regard to an expected movement behavior of the
monitored object in order to achieve a sufficiently fast response time in
the output signal. Advantageously, the perturbation time may be at most 1
s, more preferred at most 100 ms and most preferred at most 50 ms. In
particular, a value of 100 ms or below generally yields good results with
respect to the time response of the proximity switch in various
applications.

[0011] For example, a value of the perturbation time in between 10 ms and
100 ms has been found to produce an output signal of a high quality by
maintaining the above mentioned advantages in certain applications, such
as electric resistance welding. Moreover, a value of at least 20 ms of
the perturbation time may be chosen in particular for specifically
problematic applications, such as electric resistance welding at a medium
frequency, in order to efficiently suppress an entire ensemble of
occuring perturbations and still ensure the desired detection
sensitivity.

[0012] Preferably, the processing circuit is configured to generate an
intermediate signal comprising two signal states and to feed the
intermediate signal to the suppression circuit to generate the output
signal. In this way, an easier treatability of the intermediate signal
that is preprocessed in such a manner can be exploited and the design of
the suppression circuit may be simplified. Thus, an implementation of a
more complex filtering circuit that would be necessary directly at the
entrance of the measured signal can be avoided. For instance, a
comparator circuit may be applied in order to provide the intermediate
signal.

[0013] Preferably, the first signal state of the intermediate signal
indicates the absence of the object from the monitored area and the
second signal state indicates the presence of the object within the
monitored area. Preferably, the two signal states of the intermediate
signal are distinguished in the suppression circuit by means of at least
two switching states, more preferred at least four switching states. The
switching states may be defined by a switching circuit comprising
respective switches. In order to minimize the design complexity and
fabrication expense of the electronic circuit, at least two of the
switching states are preferably integrated in a single switching unit,
such as a multiplexer and/or a flip-flop or the like. More generally, the
switching circuit is preferably configured to allow a distinguished
treatment of said signal states in the suppression circuit. For instance,
the switching circuit may also be constituted by a processing unit,
wherein the switching states may be realized by a conditional programming
code. Preferably, the suppression circuit further comprises at least one
delay circuit for delaying the intermediate signal by the duration of
said perturbation time for each of said signal states.

[0014] The described suppression circuit of the proximity switch allows a
complete suppression of undesired frequencies in the output signal
ranging below said predetermined perturbation time. It is therefore
superior as compared to a conventional low pass filter which generally
only allows a signal attenuation beyond a certain cut-off frequency.

[0015] According to a first embodiment of the suppression circuit, the
same value of said perturbation time is applied for each of the two
signal states. According to a second embodiment, a different value of the
perturbation time is applied in the suppression circuit for the two
signal states. Preferably, the different values of the perturbation time
for the two signal states are fixed by means of at least one diode in the
suppression circuit. The usage of the same value or different values of
the perturbation time may be determined in dependence of the desired
application of the proximity switch and the properties of perturbing
fields.

[0016] Preferably, in order to implement an easy design of the suppression
circuit, the suppression circuit comprises at least one RC member with
components determining said perturbation time. In particular, the
elementary design of the suppression circuit may comprise a monostable
multivibrator. Preferably, the monostable multivibrator is retriggerable
in order to further improve the response behavior of the output signal.

[0017] Preferably, the polarity is reversed during a sequence of the
transmitting current pulses. The sequence of the transmitting current
pulses preferably comprises a preceding current pulse and a succeeding
current pulse, wherein the polarity of the succeeding current pulse is
reversed with respect to the polarity of the preceding current pulse.
Preferably, this is achieved by a corresponding polarity reversal in a
sequence of the voltage pulses that are applied to the coil. In this way,
the impact of perturbing fields, in particular in the lower frequency
range, can be reduced already at the entrance level of the signal to be
measured such that the effect of the suppression circuit of providing an
unperturbed output signal may be enhanced. Thus, a polarity reversal in
between a sequence of transmitting current pulses within a predetermined
time period may be used as an additional adjustable parameter in order to
optimize the detection sensitivity and perturbation insensitivity of the
output signal of the proximity switch.

[0018] Preferably, said coil, said pulse source, said processing circuit
and said suppression circuit are arranged in a housing being made of a
metal which is not ferromagnetic and which has a specific electric
resistance higher than aluminium. More preferred, said metal comprises
stainless steel. In this way, the robustness of the proximity switch can
be improved such that the electronic components located therein are well
protected.

[0019] A preferred arrangement of the proximity switch may comprise a
perturbation source that is adapted to produce a perturbation signal with
a frequency of at most 50 kHz. The perturbation source may also comprise
or consist of a direct current (DC) source, since for instance
interruptions or switch-on operations of such a DC source can also
produce perturbing fields. According to a preferred use of the proximity
switch, the proximity switch is applied in electric resistance welding.
During such a welding process, perturbing fields with comparatively large
amplitudes are produced such that an application of the described
proximity switch appears particularly advantageous.

[0020] The most common types of electric resistance welding include
resistance welding at a lower frequency of 50 Hz and at a medium
frequency of 1 kHz which is subsequently commutated to a frequency of 2
kHz. Thus, perturbing fields with a frequency of 50 Hz are mostly
produced during the resistance welding technique at the lower frequency.
In contrast, resistance welding at the medium frequency produces
perturbations with a fundamental frequency of 2 kHz and furthermore a
distinctive series of higher harmonics. According to the invention, the
described proximity switch may be used in the vicinity of both of these
welding processes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The invention will be described in more detail in the following
description of a preferred exemplary embodiment with reference to the
accompanying drawings. In the drawings:

[0022] FIG. 1 is a schematic cross-section of an inductive proximity
switch;

[0023]FIG. 2A-2D are diagrams of signals at the entrance of the
electronic circuitry of the proximity switch shown in FIG. 1 illustrating
its basic measurement principle;

[0024]FIG. 3 is a block diagram of the electronic circuitry of the
proximity switch shown in FIG. 1;

[0025]FIG. 4 is a basic circuit diagram of the suppression circuit of the
electronic circuitry shown in FIG. 3 according to a first embodiment of
the invention;

[0026]FIG. 5 is a basic circuit diagram of the suppression circuit of the
electronic circuitry shown in FIG. 3 according to a second embodiment of
the invention;

[0027]FIG. 6A, 6B are diagrams of an input signal and output signal of
the suppression circuit shown in FIG. 4 in the absence of external
perturbations;

[0028]FIG. 6C, 6D are diagrams of an input signal and output signal of
the suppression circuit shown in FIG. 4 in the presence of external
perturbations;

[0029]FIG. 6E is an idealized diagram of the voltage behavior on a
capacitance of the suppression circuit shown in FIG. 4 corresponding to
the input signal and output signal shown in FIGS. 6C and 6D;

[0030] FIG. 7 is a basic circuit diagram of the suppression circuit of the
electronic circuitry shown in FIG. 3 in a more general representation as
compared to the circuit diagrams shown in FIGS. 4 and 5; and

[0031]FIG. 8A, 8B are diagrams of an input signal and output signal of a
delay circuit in the suppression circuit shown in FIG. 7.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0032] FIG. 1 shows an inductive proximity switch 1 comprising a coil 2
connected to an electronic circuit 3. The basic measurement principle of
proximity switch 1 is disclosed in European patent applications no. EP 0
936 739 A1 and EP 0 936 741 A1 which are herewith included by reference.
As best seen in FIG. 3, the electronic circuit 3 includes a pulse source
4 for supplying coil 2 with periodic transmitting current pulses, a
bridge circuit 5 and a processing circuit 6 for generating an output
signal 9 in dependence of a change of position of a monitored object.
Referring again to FIG. 1, the electronic circuit 3 is connected to a
cable 7 which serves for transmission of the output signal 9. The coil 2
and the electronic circuit 3 are arranged in a housing 8 consisting of a
single piece of stainless steel which is not ferromagnetic and
substantially has a cylindrical shape.

[0033] FIGS. 2A and 2B depict control signals supplied by the pulse source
4. Control signal p(t) shown in FIG. 2A controls the polarity of the D.C.
voltage applied to coil 2. Control signal s(t) shown in FIG. 2B is
composed of periodical pulses S1, S2, S3 with a period T that is larger
as the respective pulse length Ts. For instance, the period T of pulses
S1, S2, S3 may be in the range of 0.1 ms, wherein the respective pulse
length Is may roughly correspond to one fifth of this period T. The
period Tp of control signal p(t) corresponds to the double period T of
pulses S1, S2, S3.

[0034]FIG. 2c depicts the current Ic(t) flowing through coil 2.
Corresponding to the periodical pulses S1, S2, S3 of control signal s(t),
transmitting current pulses Ic1, Ic2, Ic3 are present at each period T
with an increasing amplitude during the respective pulse durations Is and
a subsequently more rapidly decaying amplitude. Due to the polarity
inversion governed by control signal p(t), the second current pulse Ic2
has an opposite sign as compared to the preceding and subsequent current
pulse Ic1, Ic3. In this way, the impact of interferences with external
low-frequency fields may be reduced, as described in more detail in
European patent application no. EP 0 936 739 A1.

[0035] When a monitored object is in the zone of influence of the variable
magnetic field that is generated in coil 2 by the transmitting current
pulses Ic1, Ic2, Ic3, voltages are induced in the object leading to a
decaying current flowing in the object.

[0036]FIG. 2D represents the received voltage Ui(t) that in return is
induced in coil 2 due to the magnetic coupling with the decaying current
in the monitored object. Each of the transmitting current pulses Ic1,
Ic2, Ic3 causes a temporally shifted received voltage pulse Ui1, Ui2,
Ui3. The received voltages Ui1, Ui2, Ui3 represent a measured signal
forming a basis for the output signal 9 to be generated in the processing
circuit 6. The received voltages Ui1, Ui2, Ui3 typically exhibit a signal
strength of approximately 100 μV to 300 μV and cover a bandwith
ranging from 1 kHz to 100 kHz.

[0037] As shown in FIG. 3 in a schematic manner, the processing circuit 6
of proximity switch 1 comprises a reception window control 10, an
amplification chain 11, a comparator circuit 12 and a suppression circuit
13. Reception window control 10 is primarily used for controlling the
temporal position and duration of the received voltages Ui1, Ui1, Ui3 to
be measured. The measured signals are then amplified at the amplifier
stages 11 and further processed by means of comparator circuit 12. In
this way, an intermediate signal 19 is provided at the input of
suppression circuit 13 in digital form indicating the absence or the
presence of the monitored object with respect to the monitored area.

[0038]FIG. 4 depicts a first embodiment of a circuit diagram 13a of
suppression circuit 13. The basic functionality of circuit 13a
corresponds to a retriggerable monostable multivibrator. The intermediate
signal 19 is delivered to the input of suppression circuit 13a at the
position of transistor 14. The output signal 9 of suppression circuit 13a
is provided at the output of comparator 15. Furthermore, four switches
SW1, SW2, SW3, SW4 are provided in the suppression circuit 13a. In this
way, a switching circuit is constituted to allow a distinguished
treatment in dependence of the momentary signal state of the intermediate
signal 19. Switches SW1 and SW3 are of the type normally open (NO).
Switches SW2 and SW4 are of the type normally closed (NC). Thus, the
switches SW3 and SW4 are used to memorize the previous state of
intermediate signal 19 when a change of the signal state occurs at the
input at transistor 14.

[0039] A predetermined perturbation time Tc1, Tc2 is fixed to be 25 ms in
terms of the charging time of RC-member 16. This allows to distinguish
four different switching states in suppression circuit 13a that can be
stored by means of the switches SW1, SW2, SW3, SW4. The first switching
state corresponds to a charging period Tc1 of RC-member 16. The second
switching state corresponds to a decharging period Tc2 of RC-member 16.
The third switching state corresponds to the potential Vc being applied
to the capacitance of RC-member 16. The forth switching state corresponds
to the capacitance of RC-member 16 being at ground.

[0040] In the circuit 13 shown in FIG. 4, the perturbation time Tc1, Tc2
is chosen to be equal for both signal states of the intermediate signal
19.

[0041] According to a different embodiment of the circuit 13, a different
value of a perturbation time Tc1 may be chosen for the first signal state
as compared to the perturbation time Tc2 of the second signal state. This
can be achieved for instance as depicted in FIG. 5. The embodiment of the
suppression circuit 13b shown in FIG. 5 corresponds to the one of
suppression circuit 13a shown in FIG. 4 with the difference that a
resistance 22 and a diode 21 are connected in parallel with the
resistance 17 of suppression circuit 13b. In this way, the electrical
characteristics of these two components 21, 22 determines the difference
between the two perturbation times Tc1, Tc2.

[0042] FIGS. 6A and 6B depict diagrams of the intermediate signal 19 and
the output signal 9 of suppression circuit 13 in the absence of external
perturbations. The intermediate signal 19 shown in FIG. 6A is set to be
in the first signal state during the time periods T1 and T3 indicating
that the monitored object is located outside of the monitored area.
During the time period T2 in between, the intermediate signal 19 is set
to be in the second signal state indicating that the monitored object is
found to be inside the monitored area. The corresponding output signal 9
shown in FIG. 6B coincides with the intermediate signal 19 with two
exceptions: Firstly, the transition from the first signal state to the
second signal state after period T1 is delayed by the duration of
perturbation time Tc1. Secondly, the transition from the second signal
state to the first signal state after period T2 is delayed by the
duration of perturbation time Tc2.

[0043] Subsequently, the mode of operation of suppression circuit 13 is
exemplified on the basis of the input signal 19 shown in FIG. 6A. During
the period T1, the input at transistor 14 and the output at comparator 9
is set to zero voltage. At the end of period T1, a change of the signal
appears from the first state to the second state at transistor 14 such
that switch SW2 opens and switch SW1 closes. Switches SW3 and SW4 stay in
their current switching state. As a consequence, the capacitance of
RC-member 16 starts being charged. The duration of the signal being in
the first state during period T2 exceeds the charging time corresponding
to perturbation time Tc1. Thus, the comparator 15 tilts over and inverts
the switching state of switches SW3 and SW4. In this way, the potential
Vc is applied to the capacitance of RC-member 16.

[0044] The situation is inverted at the end of period T2, when a change of
the signal appears from the second state to the first state at transistor
14 such that switch SW1 blocks and switch SW2 is conductive. Switches SW3
and SW4 stay in their current switching state. As a consequence, the
capacitance of RC-member 16 starts being decharged via resistance 18.
Since the decharging time is sufficiently long, i.e.

[0045] exceeding the perturbation time Tc2, the comparator 15 tilts over
and inverts the switching state of switches SW3 and SW4. In this way, the
capacitance of RC-member 16 is connected to ground.

[0046]FIG. 6C depicts a diagram of the intermediate signal 19 of
suppression circuit 13 in the presence of external perturbations. The
signal 19 of FIG. 6C corresponds to the intermediate signal 19 shown in
FIG. 6A, wherein the external perturbations give rise to additional
signal variations in between the first and second state during the
periods T1 and T3. However, each of the additional signal variations has
a duration Tf substantially smaller as compared to the perturbation time
Tc1, Tc2. Thus, the charging time of the capacitance of RC-member 16 is
not sufficient during the periods Tf for achieving a tilt over of
comparator 15. Thus, the input voltage is returned to zero voltage and
switch SW2 conducts, such that the capacitance of RC-member 16 is again
connected to ground.

[0047]FIG. 6D depicts the corresponding diagram of the output signal 9 in
the presence of the external perturbations shown in FIG. 6C. As can be
seen, the additional signal variations of the intermediate signal
stemming from the perturbing fields are efficiently suppressed by means
of the suppression circuit 13. In this way, the minimum time interval
between two variations of the output signal 9 cannot go below the
respective perturbation time Tc1, Tc2.

[0048] The functionality of suppression circuits 13, 13a, 13b is further
illustrated in FIG. 6E depicting the voltage behavior of the capacitance
of RC-member 16 in the case of the perturbed input signal on transistor
14 as shown in FIG. 6C. During period T1, each of the signal variations
with duration Tf lead to an increasing voltage at the capacitance.
However, since the time of charging the capacitance is substantially
smaller than the perturbation time Tc1 the system returns each time to
its initial state before a state reversal by means of a tilt over of
comparator 15 can occur.

[0049] The situation changes at the end of period T1, when the signal
variation exceeds the value of perturbation time Tc1. Hence, the
capacitance of RC-member 16 gets fully charged by the potential Vc after
the tilt over of comparator 15.

[0050] FIG. 7 depicts a more general representation of the detailed
circuit diagrams 13a and 13b of the suppression circuit 13 of the
proximity switch 1 shown in FIG. 3. The suppression circuit 13c comprises
a multiplexer 25 which is connected to the output of comparator circuit
12 and comprises switches to distinguish in between the two signal states
of intermediate signal 19. Accordingly, the functionality of the
multiplexer 25 is provided by the switches SW1, SW2 of the more detailed
suppression circuits 13a, 13b. Thus, the switching state of multiplexer
25 depends on the momentary signal state of intermediate signal 19.
Depending on the momentary signal state, the intermediate signal 19
either passes through a first signal line 26 for the signal in the first
signal state or a second signal line 27 for the signal in the second
signal state.

[0051] Each of the signal lines 26, 27 is provided with a delay circuit
28, 29. The delay circuit 29 of the second signal line 27 is provided
with an inverter in order to take into account a reversed signal of the
second signal state. The delay circuits 28, 29 cause a temporal delay
during which a signal must be present at the respective input of the
delay circuits 28, 29 in order to allow a change of the signal to be
present at the output of the delay circuits 28, 29. The temporal delay is
fixed in the delay circuits 28, 29 as the value of the perturbation time
Tc1, Tc2, respectively. Thus, the delay circuits 28, 29 correspond to the
functionality of RC member 16 of the previously described suppression
circuits 13a, 13b.

[0052] A flip-flop 31 is connected to the output of the delay circuits 28,
29 at the end of the signal lines 26, 27. The two switches of the
flip-flop 31 are used to memorize the previous state of intermediate
signal 19 when a change of the signal state occurs at the input. This
functionality is realized by the switches SW3, SW4 of the previously
described suppression circuits 13a, 13b. At the output of flip-flop 31
the output signal 9 is provided. Moreover, the output of flip-flop 31 is
connected to multiplexer 25 via a signal line 32 in order to allow a
change of the switching position of the multiplexer 25 depending on the
momentary signal state.

[0053]FIG. 8A, 8B further illustrate the basic functionality of delay
circuits 28, 29 using the example of delay circuit 28 of the first signal
line 26. FIG. 8A shows the time evolution of a signal 35 at the input of
delay circuit 28. FIG. 8B depicts the corresponding signal 36 at the
output of delay circuit 28. During the initial phase, a first transition
from the first signal state to the second signal state occurs in the
input signal 35. The signal duration in the second signal state, however,
is smaller as compared to the value of the perturbation time Tc1. Thus,
the pulse length within the second signal state is completely suppressed
by delay circuit 28 and no change occurs in the output signal 36 of delay
circuit 28. During a subsequent phase of the input signal 35, a second
transition from the first signal state to the second signal state occurs,
with a duration that is substantially longer as compared to the
perturbation time Tc1. In consequence, a change of the output signal 36
of delay circuit 28 occurs which can be attributed to this state
transition and which is delayed by the value of perturbation time Tc1.

[0054] Thus, the mode of operation of suppression circuits 13a, 13b and
13c is equivalent, as described above with respect to the examplary
signals shown in FIG. 6A-6E. For instance, the input signal 19 shown in
FIG. 6C may be taken as an specific example to further illustrate the
mode of operation of suppression circuit 13c: During the short signal
variations in period T1 of input signal 19, no change of the output
signal 9 occurs, as shown in FIG. 6D. The reason is that the duration of
these signal variations is shorther than the temporal delay Tc1 that is
fixed by the respective delay circuit 28 of suppression circuit 13c.

[0055] The length of the signal during period T2 is substantially longer
as compared to the temporal delay Tc1 of delay circuit 28. Thus, a signal
change occurs at the output of delay circuit 28, which is delayed by the
value of the perturbation time Tc1 and then passed to flip-flop 31. The
switching states of flip-flop 31 are switched, in turn, into their
alternative switching state. This also causes the switching states of
multiplexer 25 to change via a signal transmitted by signal line 32. As a
result, the input signal 19 now passes through the second signal line 27
through delay circuit 29.

[0056] At the beginning of period T3, the input signal 19 returns to the
first signal state. The duration in which the input signal stays in the
first signal state exceeds the temporal delay Tc2 which is determined by
the respective delay circuit 29 in the second signal line 27. Thus, the
switching states of flip-flop 31 are switched again into their initial
configuration. This also causes the switching states of multiplexer 25 to
change via a signal transmitted by signal line 32. As a result, the input
signal 19 passes again through the first signal line 26 through delay
circuit 28. The following short signal variation in period T3 of input
signal 19 is again shorther than the temporal delay Tc1 that is fixed by
delay circuit 28 and therefore does not cause a corresponding change of
the signal at the output of delay circuit 28.

[0057] In this way, the minimum time interval between two variations of
the output signal 9 cannot go below the respective perturbation time Tc1,
Tc2 of delay circuits 28, 29 and external perturbations in this frequency
range are effectively suppressed.

[0058] It is to be noted that the invention is not restricted to the
embodiments described above which are for purposes of understanding and
illustration only and are not to be construed as a limitation of the
invention. In particular, the invention has been described by way of
example of a specific embodiment of the suppression circuit. As it is
known to a skilled person, a multitude of other electronic
implementations are possible for the realization of such a circuit
suppressing a signal duration lower than the predetermined perturbation
time, be it in digital or analog form. For instance, various
possibilities for providing the described switching states in the
suppression circuit are known in the art, in particular transistors,
relais, multiplexers, flip-flops or a software code of a signal
processing unit. It is preferred, therefore, that the present invention
be limited not by the specific disclosure herein, but only by the
appended claims.

Patent applications by Marc De Huu, Murist CH

Patent applications by Peter Heimlicher, Fribourg CH

Patent applications in class Using a comparison or difference circuit

Patent applications in all subclasses Using a comparison or difference circuit